In both cases, you need to ​register as an Apple developer first. (This is a free registration.)

Secondly, if you want to build the development version of GHC from the Git repositories, you also need to install the GNU auto tools (which are no longer distributed by Apple). You can get them as follows:

Thirdly, if you like to use GHC's LLVM backend, also execute: brew install llvm

NB: You need to use a binary distribution of GHC 7.4.1 (or later) as a bootstrap compiler. (Earlier versions fail with an error message complaining about the lack of /Developer/usr/bin/gcc.)

Previous versions of OS X and Xcode

Get the most recent version of Apple's Xcode tools that you can. Your OS X CD has a version on it. You may be able to download a newer version from the ​Apple Developer Connection website. You may need to sign up for a free membership in the Apple Developer Connection, and downloading may still cost a little money. In later versions of OS X (10.6 / 10.7), Apple added the "App Store". Xcode is available within the App Store for "Free".

Once upon a time Apple distributed updates to Xcode by using their Software Update service but not anymore. You must download the development tools manually. When using the App Store, it should correctly identify when you need an update for Xcode (if you downloaded Xcode from the App Store originally).

Successful builds of older GHC sources have been reported using Xcode 3.0, 2.4 and 2.4.1 on Intel Macs. Xcode 2.2.1 is known not to work out of the box on Intel Macs, and Xcode 3.0 is known not to work out of the box on PowerPC Macs (#2887). Versions prior to 3.1 may build GHC successfully, but choke on certain libraries.

Building the distribution

Install a binary GHC distribution: either from a bindist (.tar.bz2) or from an installer (.pkg).

Building using MacPorts

ghc 6.6 can be built from source using ​MacPorts on Intel and PowerPC Macs.
Follow the instructions on the MacPorts website for installing the infrastructure and port files,
then type

> sudo port install ghc

to build and install ghc. Dependencies will be built and installed automatically. There is
no need to follow the instructions above for installing readline; this is handled by
MacPorts dependency mechanism. If you want to see how the build is progressing, type

> sudo port -dv install ghc

The "-dv" flags indicate verbose debugging output. Using these can also help diagnose build problems.

If the build is interrupted or fails for some reason you must clean up before trying again. Do

> sudo port clean ghc

before restarting.

A nice feature of MacPorts is that you can put its installation directory tree anywhere.
This allows installations without administrator privileges.

Case insensitivity

The default Mac OS X files systems (HFS+) is case-insensitive and Git is case sensitive (although this is configurable). While this ususally doesn't cause any problems, occassionally a Unapplicable patch error can occur. It's possible to work around this by using Disk Utility to create a case sensitive file system and apply the patches inside of it. To do this:

Open /Applications/Utilities/Disk Utility.

Make sure that none of the images/disks on the left are highlighted/selected. If any are, <Cmd>+Click them to unselect them.

Click the "New Image" button.

Set the "Volume Format" to "Mac OS X Extended (Case Sensitive)".

Set "Encryption" to "None".

Set "Partitions" to "Single Partition - Apple Partition Map"

Set "Image Format" to "sparse disk image".

Set "Volume Size" to "Custom..." and select an appropriately large size. Sparse images only take up as much space is as needed, plus a little overhead, so it's better to overestimate than underestimate. A 30 GB sparse image with no data in it takes up ~50 MB.

Set "Volume Name" to something appropriate (e.g., "GHC").

Set "Save As" to something appropriate (e.g., "GHC Disk").

Click the "Create" button.

This creates a file with a .sparseimage extension (e.g., GHC Disk.sparseimage) at the location that was set in step 10 and automatically mounts it. The partition can be accessed through the /Volumes folder (e.g., /Volumes/GHC). This partition behaves exactly like any other Apple partition except that it's case sensitive and git can apply the patches it couldn't on the case insensitive file system. After the patches have been applied, the repository can be copied to the normal file system, the partition can be unmounted, and the sparse image can be deleted.

Building the documentation

For some reason, convincing dblatex to build the docs can be tricky. This worked for me: